1. Field of the Invention
The present invention relates to a surface-mount negative-characteristic thermistor including plating films arranged on surfaces of external electrodes and suitable for being mounted on a substrate.
2. Description of the Related Art
In recent years, surface-mount electronic components have been required, and there have been advances in the fabrication of negative-characteristic thermistors in chip form, the thermistors having negative resistance temperature characteristics. In such negative-characteristic thermistor chips, for example, Japanese Unexamined Patent Application Publication No. 2004-104093 discloses a laminated negative-characteristic thermistor having a ceramic body containing Mn, Ni, and Al and having satisfactory reliability and a small change in properties with time.
The laminated negative-characteristic thermistor disclosed in Japanese Unexamined Patent Application Publication No. 2004-104093 will be described below with reference to FIG. 3. FIG. 3 is a schematic cross-sectional view of the laminated negative-characteristic thermistor disclosed in Japanese Unexamined Patent Application Publication No. 2004-104093. The laminated negative-characteristic thermistor 11 includes a ceramic body 14 having a plurality of ceramic layers 12 having negative resistance temperature characteristics; and a plurality of internal electrodes 13 disposed along interfaces between adjacent ceramic layers. External electrodes 15 are disposed on ends of the ceramic body 14 and connected to the internal electrodes 13. The ceramic layers 12 are each composed of a ceramic material containing Mn and Ni as main constituents and Al as an additive. The internal electrodes are each composed of Pd. The external electrodes are each composed of Ag.
The laminated negative-characteristic thermistor 11 is produced by a known method described below. An organic binder is mixed with a ceramic powder to form a slurry. The slurry is formed into ceramic green sheets by a doctor blade method or the like. A paste for forming internal electrodes and mainly containing Pd is applied to the ceramic green sheets by screen printing to form electrode patterns. The ceramic green sheets with the electrode patterns formed thereon by screen printing are laminated, held by the ceramic green sheets on which the electrode patterns are not formed by screen printing, and press-bonded to form a laminate. The resulting laminate is degreased and sintered to form the ceramic body 14 in which the internal electrodes 13 and the ceramic layers 12 are alternately laminated. A paste for forming external electrodes and containing Ag is applied to both ends of the resulting ceramic body 14 and baked to form the external electrodes 15.
When the resulting ceramic body 14 provided with the external electrodes 15 is surface-mounted on a substrate, soldering is usually performed. During soldering, the external electrodes 15 are melted and become eroded by solder, i.e., solder leaching may occur. To prevent solder leaching or to ensure solder wettability, plating films composed of Ni, Sn, or the like are generally formed on the external electrodes before soldering.
However, during forming of the plating films on the external electrodes 15 disposed on the ends of the ceramic body 14, a plating solution is disadvantageously in contact with the ceramic body 14 and erodes the ceramic body 14. The erosion of the ceramic body 14 due to the plating solution disadvantageously reduces the strength of the ceramic body 14. In particular, the ceramic body 14 composed of the material for a thermistor, the material containing Mn and Ni as main constituents and Al as shown in Japanese Unexamined Patent Application Publication No. 2004-104093, has excellent reliability and only small changes in properties over time, whereas the ceramic body cannot sufficiently prevent erosion due to the plating solution. Thus, to prevent the erosion due to the plating solution, a technique for providing an insulating protective layer, such as a glass layer, on a surface of the ceramic body 14 may be employed (for example, Japanese Unexamined Patent Application Publication No. 6-231906).
However, even when the glass layer is formed on the surface of the ceramic body 14, the plating solution may penetrate into the ceramic body 14 through minute cracks and pinholes present in the glass layer. Hence, the erosion of the ceramic body 14 is not sufficiently prevented. Furthermore, in the case of the formation of the glass layer, it is necessary to perform an additional step of forming the glass layer on the surface of the ceramic body 14, disadvantageously complicating a production process.